| Due to the development of social economy,the increase of population and motor vehicles,the increase of anthropogenic pollutants,the haze pollution in the central and eastern part of China has become increasingly serious in the past three decades.When haze pollution occurs,the main pollutants in the atmosphere show that the concentration of fine particulate matter(PM2.5)is seriously exceeded the standard.Despite these research outcomes,the formation mechanism and evolution of fine particulate matter have not been fully understood,particularly with respect to identifying the major contributors to the explosive growth of PM2.5 mass concentration.At present,the overall temporal pattern of explosive growth of PM2.5 events was rarely been studied.Therefore,a long-term online observation data and more detailed comparison of emission and meteorological conditions in different years are essential for better elucidating the impacts of emission changes and meteorological conditions on haze pollution.In order to study the aerosol evolution and meteorological factors of fine particle explosive growth(FPEG)events in Beijing-Tianjin-Hebei region,a 5-year(from January 2013 to December 2017)online observation of PM2.5 was employed in Beijing,Tianjin and Shijiazhuang in this study.We established a definition method for FPEG events by the concentration of PM2.5 increasing rapidly from tens of micrograms per cubic meter(initial concentration less than 75?g/m3)to hundreds of micrograms per cubic meter(peak concentration greater than 150?g/m3)within less than several hours(usually 9h).According to their duration,FPEG events were further divided into three types:3-h,6-h,and 9-h events.This study discusses inter-annual and seasonal variations in FPEG events and combines the observation data of gaseous precursors,water-soluble inorganic ion and meteorological elements in the same period of FPEG events,focusing on the characteristics of pollution and meteorological causes of FPEG events.Based on online observations from January 2013 to December 2017,the largest number of FPEG were observed in Beijing(132),followed by Shijiazhuang(125)and Tianjin(71).The yearly variation trend of FPEG events is consistent with that of atmospheric fine particulate matter,showing a downward trend year by year.9-h events were the most abundant in Beijing and Tianjin,while the 6-h events were the main event in Shijiazhuang.Normal haze episodes also showed a downward trend year by year,while clean days showed an upward trend year by year.In terms of seasonal variation,the occurrence frequency of FPEG events in autumn and winter was higher than that in spring and summer,normal haze episodes mostly occurred in winter,while clean days were mostly in summer.The yearly variation in each season were analyzed during the observation period,the higher concentrations of PM2.5 had a higher SO2 concentrations in Tianjin,while the higher PM2.5 concentrations in specific years were all in accordance with simultaneously higher concentrations of CO in Beijing,as well as the all events had higher RH and lower winds.While the higher concentrations of PM2.5 in Shijiazhuang had a high concentration of SO2 and NO2.In terms of seasonal variation,the PM2.5 concentration of FPEG events showed high in winter and autumn.The higher concentrations of SO2 of three types of FPEG events observed during wintertime suggested coal consumption for heating in the north in winter.In addition,lowest temperature,highest atmospheric pressure,lower atmospheric boundary layer,weaker turbulent motion and air convection,coupled with the occurrence of inversion phenomenon in winter,increased the stability of the atmosphere.Therefore,the occurrence frequency of FPEG events is relatively high in winter.The transportation of the southeast plays a more important role in the formation of 3-h events,while local emissions contribute more to 9-h events and normal haze episodes.In addition,the formation of 6-h events originated from the combined contributions of both local emissions and regional transportation.According to the analysis of regional transport and local emissions.These results suggest that regional transport is important for FPEG events in Beijing.Meanwhile,there was an apparent decrease at 9-h events in Beijing.The results indicate that the duration of the stagnation case and regional transport duration is shortened.Different from Beijing area,local emissions have a greater contribution to the FPEG events in Tianjin.At the same time,there was an apparent decrease at 6-h events but not obvious variation at 3-h and 9-h events for stagnation.The results indicate that the duration of FPEG events is extended for stagnation in Tianjin.According to the analysis of the change of gaseous precursors for FPEG events,it is found that the traffic emission sources have made an important contribution to FPEG events under stagnant and regional transport conditions in Beijing and Tianjin.PSCF analysis shows that the pollution sources in Beijing and Tianjin mainly come from the southern region under regional transport conditions.SNA(SO42-,NO3-and NH4+)dominated 6-h events under stagnant and regional transport conditions in Beijing.According to the study of cases in 6-h event under the stagnant conditions in Beijing,it can be concluded that primary emissions and secondary transformation play a vital role in this event. |
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